1. Field of the Invention
The invention relates to power amplifiers in general and particularly to a power mixer array that employs a plurality of power generation elements.
2. Description of Related Art
Non-constant envelope modulation schemes have become more commonplace in cellular applications due to their higher spectral efficiency. Linear power amplifiers (Pas) are usually used to transmit these signals faithfully at the expense of the power efficiency. Separate processing of amplitude and phase information (e.g., EER or polar modulation) has been proposed as way of improving the system efficiency. See L. Kahn, “Single-sided transmission by envelope elimination and restoration,” Proc. IRE, pp. 803-806, July 1952. However, many of these schemes require an efficient high-power low-frequency supply modulator to reconstruct the amplitude information. This can be done, for instance, using a DC-to-DC converter with its own limitations in efficiency, bandwidth, and chip area (because of the requirement for an external inductor).
Even for an ideal supply modulator, the amplitude dynamic range of the power amplifier itself is limited by the gate to drain feed-through. For example, as described in S. Hietakangas, et al., “Feedthrough cancellation in a class E amplified polar transmitter”, European Conference on Circuit Theory and Design, pp. 591-594, August 2007, a 10 dB change in the supply results in 5° phase shift at the output. Although digitally modulated polar power amplifiers described by A. Kavousian, et al., in “A Digitally Modulated Polar CMOS PA with 20 MHz Signal BW”, ISSCC Dig. Tech. Papers, pp. 78-79, February 2007 has been shown as a possible solution at lower power levels, its implementation in a wideband watt-level fully-integrated setting with low spurious and out-of-band emission faces practical challenges such as number of required bits and layout symmetry.
As shown in FIG. 1, various approaches that have been tried suffer from different deficiencies. For example, class A amplifier and class AB amplifier implementations are faithful, traditional implementations. However, the peak efficiency is not good, and the efficiency drops abruptly with decreased output power. In the Doherty implementation, matching is difficult, and there are issues relating to area. In DC-DC implementations, there are issues relating to area, the need for external inductance, overall efficiency, and back-off efficiency. In some digital implementations, the use of high resolution RF-digital-to-analog converters (DAC) generally leads to a larger area resulting from inefficient layout, and it becomes extremely difficult to realize the same impedance seen by each amplifier. Digital implementations have not demonstrated watt-level power amplification. In sigma-delta implementations, one experiences out of band noise.
There is a need for a wideband watt-level power amplifier that provides good linearity and high efficiency.